DRV8461SPWPR datasheet(41/117 Pages) TI

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The Auto-torque algorithm implemented in the DRV8461 improves system efficiency by dynamically changing

the output current according to the load torque. Whenever the load torque is low, the output current is lowered to

reduce resistive losses; and when the load torque goes up, the output current increases immediately to prevent

motor step loss. This concept is shown in Figure 7-22. As a result of improved efficiency due to auto-torque, the

system runs at a lower temperature, which extends the lifetime of the components. Auto-torque can also enable

the use of cheaper and smaller sized stepper motors.

Power Saving

Op mal Current

Output Current

Time

Maximum Torque

Figure 7-22. Power Saving with Auto-torque

The auto-torque feature is enabled by writing 1b to the ATQ_EN bit.

7.3.12.1 Auto-torque Learning Routine

In a stepper motor system, the total power delivered by the power supply goes into providing for the torque

requirement of the load and into power losses such as resistive losses caused by motor winding resistance and

driver ON resistance. This is represented by Equation 11:

Power delivered by supply=Constant losses+ τ×ω

(11)

where τ is load torque and ω is motor speed.

From Equation 11, we can observe that when the load torque increases, the power delivered by the supply

increases as well. The auto-torque algorithm obtains information about the load-torque by monitoring the power

delivered by the supply. The constant losses are represented by the ATQ_LRN parameter, and the ATQ_CNT

parameter represents the power required to support the load torque.

For any given motor, ATQ_LRN is directly proportional to the coil current. This can be expressed by Equation 12:

ATQ_LRN= k×IMVVM

(12)

where IM is the motor current, VVM is the supply voltage to the driver and k is a constant. Equation 12 gives

a linear relationship between the ATQ_LRN and the motor current. The auto-torque learning routine learns

ATQ_LRN values at any two currents at no load, and then uses this relation to interpolate ATQ_LRN value at

any other current.

The ATQ_CNT parameter represents the component of the delivered power that supports the load torque. This

relation can be expressed by Equation 13.

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DRV8461

SLOSE98 – DECEMBER 2022

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